In general, an electro-explosive device (EED) receives electrical energy and initiates a mechanical shock wave and/or an exothermic reaction, such as combustion, deflagration, or detonation. The EED has been used in both commercial and government applications for a variety of purposes, such as to initiate airbags in automobiles or to activate an energy source in an ordnance system.
With reference to FIG. 1, a typical EED 10 comprises a thin resistive wire or bridgewire 12 suspended between two posts 14, only one of which is shown. The bridgewire 12 is surrounded by a flammable compound 18, commonly referred to as a pyrotechnic mix. To initiate combustion of the pyrotechnic mix 18, a DC or very low frequency current is supplied through lead wires 16 and posts 14 and then through the bridgewire 12. The current passing through the bridgewire 12 results in ohmic heating of the bridgewire 12 and, when the bridgewire 12 reaches the ignition temperature of the pyrotechnic mix 18, the pyrotechnic mix 18 initiates. The pyrotechnic mix 18 is a primary charge which ignites a secondary charge 20, which in turn ignites a main charge 22. The EED 10 further comprises various protective elements, such as a sleeve 23, a plug 24, and a case 26.
Although the EED 10 is a well known device, the electromagnetic environment in which EED's operate has changed dramatically over the past four decades. One change that has occurred in the operating environment for the EED's is that the EED's are being subjected to higher levels of electromagnetic interference (EMI). The necessary operation of high power radar and communication equipment in the proximity of EED's, such as in an aircraft carrier flight deck, has resulted in a typical operating environment that includes high intensity electromagnetic fields. The EED which initiates an airbag in an automobile may be subjected to severe EMI during the normal life-span of the automobile. Thus, EED's are being subjected to high levels of EMI in both military and non-military environments.
The high intensity radio-frequency (RF) fields which present a serious EMI problem can couple electromagnetic energy either through a direct or indirect path to an EED and cause accidental firing. Electromagnetic energy may be coupled directly to the EED when RF radiation is incident on the EED's chassis whereby the EED acts as the load of a receiving antenna. The electromagnetic energy may alternatively be coupled indirectly to the EED when RF induced arcing occurs in the vicinity of the EED and is coupled to the EED, such as through its leads. An RF induced discharge can occur whenever a charge accumulated across an air gap is sufficient to ionize the gas and sustain an ionized channel.
The EED's which are located in the vicinity of intense RF fields, such as naval surface ships, may receive signal components due to rectification of RF radiation. The RF radiation can be rectified, for instance, due to simple metal contact diode action, which is generally caused by corrosion of contacts or incorrectly connected fasteners. The rectified signal may have components that are at much lower frequencies than the source RF radiation and may also contain a DC component, any of which may couple to the EED and cause accidental ignition. The RF radiation may be rectified in many environments in which an EED may be found, including an automotive environment where large currents or voltages are switched very quickly thereby producing high levels of noise.
Another manner in which an EED may be accidentally discharged is by the coupling of an electrostatic discharge (ESD) to the EED. An ESD is characterized as a signal which is of a high voltage and fairly low energy. While the energy of the ESD is usually insufficient to cause any significant ohmic heating of the EED, the high voltage can create a sufficiently large electric field between the input pins of the EED to ignite the pyrotechnic mix.
One approach to protect an EED from EMI is to install one or more passive filters. Several standard types of passive filters exist which can be utilized to attenuate stray RF signals. These filters can usually be classified as either L, Pi, or T types, or as combinations of the three types. The L, Pi, and T type passive filters, which are respectively illustrated in FIGS. 2(A), (B), and (C), have traditionally been used as a first measure of eliminating EMI problems.
Conventional passive filters being used with EED's, however, have several disadvantages. A conventional filter consists of a combination of inductors, capacitors and/or other lossy elements, such as resistive ferrites. In general, the performance of the filter is directly proportional to the number and size of the elements used in its construction. Thus, a filter can be designed to attenuate a signal to a larger extent if the size of the inductors, capacitors and ferrite sleeves are all increased. Also, a filter having a greater number of stages will generally have an improved performance. The size of the filter, however, is often limited by the amount of available space. As a result, it may not be possible to add a filter to an EED or the filter which can fit within the available space may be ineffective in protecting the EED from EMI.
The filters are usually constructed from standard passive components assembled on a printed circuit board or hard-wired within a metal chassis. A typical example of an RF filter 30 is shown in FIG. 3(A). The RF filter 30 comprises, inter alia, a ceramic capacitor 32 and a wound torroidal inductor 34. As shown in FIG. 3(B), the ceramic capacitor 32 has a plurality of electrode layers 38 separated by a ceramic dielectric material 36. As should be apparent from FIG. 3(A), the size of the capacitor 32 and inductor 34 render the filter 30 too large for many applications, such as with weapon systems where space is especially limited. Therefore, a need exists for a small sized EED which is adequately protected from EMI.
In addition to the constraint of available space, the cost of the EED and filter can also limit the size of the filter. The cost of each filter is directly related to the number of capacitors, inductors, and other elements forming the filter. Even though some filters may have only a few components, the cost per unit price in assembling the filter may be relatively high in comparison to the cost of an EED. Thus, with a large scale production of EED's and their associated filters, the overall increase in cost can become quite substantial.
A further disadvantage to passive filters is that they are unable to filter out many low frequency signals which can cause accidental firing of the EED. Because the signal for firing an EED is a DC signal, the conventional filters are designed to freely transmit DC and other low frequency signals. These filters, therefore, are unable to attenuate the low frequency signals due to rectification of RF signals as well as other low frequency or DC signals.
Even with a filter that can effectively filter many types of EMI, the EED is not completely safe from accidental firing. In a conventional filter system, the filter and EED are essentially two separate components. With reference to FIG. 4, a non-propagating magnetic field B may induce an emf via closed loop induction. The emf is proportional to .omega.AB, where B=.mu..sub.o H, A is the cross-sectional area, and .omega. is the frequency of the magnetic field B.
The EED can be further protected from EMI by shielding. The shielding of an EED, however, is effective only if construction of a barrier and operational procedures can guarantee the integrity of the shielding structure. When a large number of EED's are manufactured, it becomes likely that some of the EED's will have defective shielding structure. Thus, shielding of the EED is not the best approach in protecting the EED.
Another device designed to protect an EED from accidental firing is a spark gap arrester. The spark gap arrester is used to reduce the chance that an electrostatic discharge (ESD) will produce an accidental firing and is essentially comprised of two conductive electrodes separated a precise distance, thereby defining an air gap. When the strength of an electric field developed across the conductors exceeds the dielectric strength of the air, a breakdown occurs and excess electric charge is free to flow across the air gap from one conductor to the other conductor. The conductor which receives the excess charge is typically connected to ground so that the charge is directed away from any sensitive elements in the EED.
A spark gap arrester relies upon precise spacing of electrodes to assure that a static discharge is shunted to the ground. The mechanics of constructing the precise air gaps can involve expensive manufacturing techniques. As a result, a spark gap arrester can significantly increase the cost of an EED.
The spark gap arrester may also be destroyed during installation and handling of the EED. A typical spark gap arrester is a discharge disc or sheet having a central opening through which lead wires can extend. A thin electrically conductive layer is in contact with the casing of the EED but is out of contact with the lead wires by the precise air gap. If the lead wires are bent, such as during assembly, the effectiveness of the spark gap may be severely hampered.
In order to reduce the sensitivity of an EED to stray signals, the total energy of the firing signal which is necessary to ignite the EED may be increased. As a result, low level stray signals can be conducted through the bridgewire without causing any ignition and only the higher level firing signal would have sufficient energy to ignite the EED.
A higher magnitude firing signal, however, is not always desirable. An EED typically has an initiation system which supplies the EED with the firing signal. The initiation system typically has a capacitor which stores the charge necessary for generating the firing signal. If the energy of the firing signal is increased and voltage remains constant, the size of the capacitor must also increase. Because of the larger capacitor, the cost of the initiation system substantially increases. Thus, by decreasing the magnitude of the firing signal, the cost of the EED and initiation system can be reduced.
It is also desirable to have a lower firing signal when the amount of available power or energy is limited. For instance, many automobiles are presently being manufactured with dual air bags, each of which requires a separate EED. Future designs of automobiles may have five or more airbags and may additionally employ EED's to actuate seat belts in the event of a collision. With the larger number of EED's that will likely be in an automobile, the magnitude of the firing signal should be as small as possible.
In the automobile environment, an airbag must be activated as quickly as possible in the event of a collision in order to maximize the amount of protection provided to the occupant of the vehicle. The EED which activates the airbag must therefore be able to ignite quickly, yet cannot be accidentally ignited with stray RF or with an ESD. Further, as described above, the EED should additionally be activated with a low energy firing signal. It has been difficult in the industry to produce an EED which can be activated quickly, which is insensitive to RF and to an ESD and is inexpensive to manufacture, and which is ignited with a low energy firing signal.
The use of an EED in an automotive environment presents other difficulties as well. For instance, the EED commonly used today to activate automotive airbags typically uses lead-azide as a primary charge. Lead-azide is an extremely explosive material and produces a fast travelling shock wave when ignited. Due to the highly explosive nature of lead-azide and the magnitude of the shock wave produced upon explosion, a steel mesh must necessarily be placed around the EED to prevent the shock output of the EED from rupturing the airbag. The high strength steel mesh, however, complicates the manufacturing process and adds further cost to the EED structure. A need therefore exists for a lower cost EED which does not require the use of a primary explosive.
The sensitivity of an EED also may be lowered with the use of a ferrite bead. When a hollowed ferrite bead is placed over a wire, the ferrite bead will pass the DC firing signal but will present an impedance that increases with frequency. Thus, with EMI, the ferrite bead will present an impedance to these signals which will thereby convert the electromagnetic energy from the signals into heat.
The effectiveness of a ferrite bead is rather limited. As the intensity of the stray signal increases, the temperature of the ferrite bead rises and, at a certain temperature, the ferrite bead loses its magnetic characteristics. Once the ferrite bead becomes too hot, the EMI is no longer converted by the ferrite bead into heat but is instead coupled to the EED, possibly igniting the EED. Thus, at higher signal levels, the ferrite bead is unable to divert the EMI away from the EED.